Method of producing acid bessemer steel or thomas steel low in nitrogen



Patented Apr. 5, 1949 METHOD or PRODUCING ACID BESSEMER STEEL on THOMAS STEEL Low IN NITRO- GEN Wilhelm Ekman and Giista Erik Hildebrand Hildebrand, Fagersta, Sweden No Drawing. Application June 22, 1945, Serial No. 601,070. In Sweden July 10, 1944 There is a clear difference in certain properties between acid Bessemer or Thomass'teel produced a in a converter (hereinafter called converter steel), on the one hand, and steel produced according to other methods,- e. g. Martin-steel, on-the other hand. It has appeared from comparative tests that the higher nitrogen content of the converter steel occasions this difference in the properties also in steel with the same chemical analysis of the other constituents. The high nitrogen content depends on the method of production. Of course, a strong absorption of nitrogen takes place in the process as hitherto usually carried out when the air current is pressed through the steel at the blowing. This is clear from nitrogen analyses showing the following values: Martinsteel 003-0.00'7% nitrogen, converter steel 0.010-0.030% nitrogen. I

The special properties which the high nitrogen content gives the converter steel over e. g. Martinsteel, are for example: greater strength (about '7 kgs./mm. higher yield limit, greater ageing liability, and greater hardnes and brittleness after cold treatment. The hardeningproperties (are also influenced by the nitrogen content.

Under certain circumstances, e. g. in the production of some kinds of tool steel, automatic steel, tubes and railway rails, some of these properties might be desirable. However, as a'rule they are not desirable. In the normal production of mer chant iron the high nitrogen content of the converter steel is most often detrimental, particularly with regard to the fact that-the nitrogen causes a hardness and brittleness of the steel after cold treatment which is greater than of other steel, e, g. Martin-steel, after the same degree of cold treatment; this has appeared to be a great disadvantage for the converter steel, inter alia in cold-drawing of wire and deep-pressing of sheet metal. I

However, it has become apparent that a converter steel, produced with a lower nitrogen content than normally, in the way according to this invention does not show the said specific properties but is completely comparable with e. g. a prime Martin-steel.

The problem of lowering the nitrogen content of the converter steel has on a larger technical scale hitherto been solved in such a way that after finished blowing the converter steel has been conveyed to another steel production furnace, generally an open hearth or electric steel furnace, and been subjected to a further refining in the said furnace. Among other things a lowering of the nitrogen content has been obtained in this second step of the duplex process.

It would, of course, be of great economical importance if such a duplex process could be avoided and a steel with low nitrogen content could be produced directly in a converter. Various sug- 4 Claims. (C1. 75-51) gestions of solving this problem have alreadybeen made earlier. For 1 example, it has been sug- 'gested to effect the blowing with an oxidizing gas mixture with low nitrogen content,- or to bring the melt to boil by adding oxidizing matter after finished or almost finished blowing whereby the nitrogen content would be reduced.

The nitrogen content of a steel which'is in equilibrium with a gas phase, is directly proportionalwith the square root of the partial pressure of the nitrogen in the gas mixture. In order to prevent or delay the absorption of nitrogen in the steel by changing the composition of the gas phase or simply obtain a lowering of the nitrogen conpartial pressure for nitrogen in the gas phase can be obtained by influencing the composition of the blast air admitted into the converter prior to the blast contacting the steel, which can be efiected e. g. by enriching the blast air with oxygen. 7

- The object of the present invention is to influence the composition of the gas phase not until it has contacted the steel by adding an oxidizing agent, e. g. iron ore, in the converterone or more times in the course'of the blowing. "However, in order that a denitration or at least a considerable delay of the nitrogen absorption shall be obtained by such addition it is necessary that according to the invention the supply of the blast air or of the blast air gas mixture at least after the addition is arranged in such a way that the blast air passes only through a certain part of the melt whereas the other part is not subjected to the direct influence of the blast air or of'the blast air gas mixture.

Several methods of efiecting a partial blowing through according to the invention are possible. In a converter provided with the bottom hitherto used this blowing through is obtained by such tilting of the converter that the larger part of the meltlies on the furnace wall and only a smaller part is in contact with the furnace bottom and thus is flowed through by the blast air. It'may also be proceeded in such a way that the blast air i admitted only through part of the furnace bottom or through the wall of the furnace.

By addition of oxidizing agent substantially silicium, manganese, carbon and iron and in a portionally large.

3 basic methodalspphosphorusz-areioxidized. Only in the'acar-bon oxidation the reactioniproduct is a gas, by the supply of which to the gas phase a reduction of the partial pressure of the remaining components, among themthat of the nitrogen, can be obtained. Itisantadvaintageous, al-' though not determinant factorof'th'e process that the contents of other substances than carbon and iron are held as low as caniberde'em'edfsuitable from other points of view. For theiri*reaction;-

products are not included in the gas phase,,and thus they do not contribute to lowering 'the partial pressure of the nitrogen. Particularly in the acid Bessemer method it is moreover :desiralble, although not necessary, that the contents of these substances are low as thereby the boiling which a--condition for the denitration or the delay-of the nitrogen absorption, more quickly gets into 1 working-orderand its intensity becomes stronger.

At addition of oxidizing agent at normal blow-;-, ing, i. e.-when. the blastx-air flows practically through the whole melt; no denitration is; ob-

itained'in spite of the addition, possibly only a :slight reduction'of .the nitrogen absorption rate.

. The explanationofit is that the volume of the :f'carbon monoxide formedper .unit of time-at'the I oxidation is slight in comparison with the :volumeof'the blastairadmitted to thesteel bath ,duringthe same time :and 0f .the 'carbon'mon- .roxide formed: thereof. 1 .The loweringof the partial .pressure'of the :nitrogen' in the gas phase t rthen becomes too small to considerably influence theinitrogen content of. the steel. yan already known suggestion :oflowering e the nitrogencontent at Thomas blowingvby ore addi-s :tion. during the dephosphorizing'period does not "bring" about: any considerablexlowering of the pnitrogen content.

Consequently,

v.However,- when theaddition of the oxidizing agent is combined with a'partial'blowing through according to the invention things are --diflferent,

.for in this case two oxidizing processes which are rdi-fierent but which merge :in one another, will take place in the melt. Part of the melt is oxiwdized by blast air'as well as by the specially added ,1

' oxidizing. agent. :In another part through which the blast air does not flow, the oxidation iscaused substantially by the specially added oxidizing iniagen't.

There is, of course, nosharp limit-between these-parts.

lnqthe first mentioned part of the melt the ..;nitrogen content'of the "gas phaseis relatively high, and there are thus factors existent :to render the absorption of nitrogenfeasible In -the other part ofthe melt the 'gasphasezsubstantially consists of carbon monoxide.

:tration: can take place'in the said phase. .In order-to 'obtain low nitrogen contents it should consequently be seen to it that that partofthe Thus denimelt to which no blast air is admitted, is pro- However, the other part in which the blast air passes, must not be too small,

- either, because insuch a case the oxidation rate becomes too low. .Moreover, .as regards the specially added oxidizing agent its oxidizing in- .=fluence is-reduced if thatagitation of the melt which is caused by the blast air fails to-comeon.

.At theme of someoxidizin'g agents, e. g.xiron tore, considerable heatiquantities are required for ".fl'ihe :reduction 1 of the oxidizing agent. 1This .heat

demandzwcan -be covered; according to earlier known .methods, blast:v air being admitted also above the surface of the bath, 1. e. by blowing in ?an oblique direction or via special twyers in the furnace walls. This can also be effected by adw'mi'ttingf'bla'st.-'air.;lto"the steel bath at such a low depth ben'e'athits surface that the oxygen of the blast air has no time to be consumed during the 1-=passage1through" the steel bath. The oxygen whi'ch' ;in some of these ways is admitted above the surface of thesteel bath, partially combusts ,itnerormed carbon monoxide within the converter and gives a su'fiicient heat quantity for the abovementioned reduction.

By the method according to the invention a converter steel canbe. produced-.whichthas the same-properties asva Martin-steel of thecorre- :spondinganalysis. At *tests made it. has been possible to reducesthe nitrogen-content ofthe converter steel torsuch values as are character- :istic of normal Martin-steel.

. The method according to the invention cam of I course, be used with. advantage :also in such a method inwhichtan oxidizing gas mixture poorer in nitrogen is blown *through instea-dof air.

Having now described ouriinvention; what we -,claim as. new and-desire "to secure by Letters Patentis:

.-1 1; .In'atmethod of producing steel? by blowing a metal melt in'a converter with-a continuous'blast of a'i-gas -containing :air but being otherwise. free a ofi-nitrogen-compounds, the steps. of adding to. the

melt in the converter prior .torthe scompletion-bf the. blowing, a nitrogen-free oxidizing agent, and

ethentcontinuing the blowing without interruption,

*while retaining the entire melt in the converter,

.-in such amanne'r that-the gas. flows through-part .-ofthemelt-only sosthat final oxidation, withxsimultaneousL denit-ration, is eifectedby the added oxidizing agent, the part 'of the'melt traversed by --the-blast gas after additionof .the oxidizingagent :being proportioned to maintain a sufiicient rate of oxidation.

- 2.-=-A-m'ethod-asclaimedinclaim Lin which the blast gas is air.

-- 3.-.-A-met;hod as claimed in claim 1-' in whichthe blast gas isanair-gas mixture.

GUSTAV. ERIK JHILDEBRAND IH'ILDEBRAND.

Number 4.2-1 A method as claimedin claim 1 in which iron ore is used as 'an oxidizing agent.

REFE-RENCES' CITED The follo'wing references are of 'reco'rdin the file of this patent:

' UNITED" STATES PATENTS v 'Name Date 4 51,401 {Bessemer Dec. 5, 1865 86,859 Phillips Feb. 9, 1869 193,551 Reese -J-u1y 24, I877 -"28'7,842 McDOn'ald Nov. "6, 1883 T553375 Sherk etaL 'Fb. 4,1896 2,244,385 Brassert "June 3,1941 2,333,654 'Lellep Nov. 9, 1 943 I 2,390,346 Bayer Dec. 4, 945

OTHERREFERENCES V ...'I-'l'ie*Journa-l of: theeIron and Steel' Insti'tute LCBritishlyo'lumeCVILNo. 1,1923, pages-665, 1666. 

